He is voiced by Yūki Kaji, who also voices Clemont in Pokémon.
True to his persona, Theodore is a fiery passionate duelist who views dueling as a way to regain himself, after losing a part of himself because of the experiments done to him by the Hanoi Knights. He is brave, smart and fierce during duels, but regrets that he did not go after the Hanoi Knights like Playmaker did. Even though he knows that the Hanoi Knights are gone, Theodore sees SOL Technologies as being equally culpable to the suffering caused by the Hanoi Project. As a result, Theodore has a deep hatred towards the Hanoi Project for causing the death of his parents, even going as far as blindly challenging Varis to a duel despite the latter's genuine interest in working together. Theodore views The Gore, Blue Angel and Playmaker as heroes for stopping the Tower of Hanoi, and respects them, to the point where he learned their real world identities (even though he is bad at programming and hacking). He becomes thrilled when he gets to Duel them, regardless of their intentions. Occasionally, Theodore shows a lack of confidence in himself and what he is doing, so he counts on Flame to give him advice on his next move.
After moving to Den City, Theodore has become polite and less rude. As a result of his torturous experience on the Hanoi Project, Theodore had a crippling fear of "Despair from the Dark" a monster that would regularly cause him nightmarish times, both figuratively and literally and that became the symbol of every torment he endured. Thanks to Flame, he was able to overcome his trauma and no longer daunted by the Zombie Monster.
Theodore has also shown to have a sense of justice as he vowed to defeat Windy in order to avenge his partner and when he promised to defeat Bohman and Lightning to avenge Kolter.